Replace table interpolation by two line fit for CNS

We used a table lookup for CNS equivalent times. Turns
out the log of this table falls pretty much on a straight
line for po2 <= 1.5bar. We now fit this tabel two two
lines, one for <= 1.5 bar and one above. This four
parameter fit has half the sum of errors squared
than the five parameter fit using a fourth order
polynomial.

Fitting the log has the advantage that this never
crosses 0, which would have the bad effect of
resulting in negative CNS values as we divide
by the table value.

We don't adopt a maximum pO2 cut-off for the CNS calculation
but rather live with the large values that the interpolation
formula produces when extrapolating.

Signed-off-by: Robert C. Helling <helling@atdotde.de>

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